1. Field of the Invention
This invention relates generally to an element useful for recording information. More particularly, the invention provides an element having a layer capable of being thermally deformed, during recording, in such a way that deformations occur at a high packing density; i.e., a large amount of information is recorded per unit area of the layer, and yet this information is capable of being read out with a high signal-to-noise ratio and few imperfections or dropouts. The element is therefore useful for the recording of video information. In another aspect, the invention relates to a method for writing and reading information on the element of the invention.
2. Description Relative to the Prior Art
Elements for recording information by thermally altering the physical structure of a material are known. U.S. Pat. No. 3,475,760 describes an element for recording information as a thermal deformation pattern. The element comprises a layer of a solvent-coated thermoplastic material on a support. The patent teaches that some thermoplastic materials, modified to have "high" absorption at the wavelength of a recording laser beam, are thermally deformed by that laser beam so that some of the thermoplastic material is displaced from the area illuminated by the beam and forms ridges along the edges of such area. This deformation pattern retains its shape after the laser beam is "removed", and thus it is possible to read the deformation pattern by projecting the pattern onto a viewing screen as a Schlieren image and viewing the image through Schlieren optics.
Similar disclosures are found in Belgian Pat. No. 858,636 and Japanese Patent Publication No. 24556/1971.
None of these patents or publications, however, teaches or suggests materials which are useful in forming a recording element with high-quality, high information-density recording capabilities. More specifically, this prior art does not suggest materials which are capable of being thermally deformed by a laser beam to provide an information record, wherein each discrete bit of recorded information is a deformation having a size less than 1.5 microns (measurement of deformation size is discussed below), and wherein such information is read back with a signal-to-noise ratio greater than or equal to 40 decibels. Such a recording element is very desirable at it allows one, for example, to manufacture inexpensively a 110-mm circular disc which is useful in recording at least one half hour of real-time video programming per disc side.
Newer developments, such as those disclosed in U.S. Pat. No. 4,023,185, have provided elements and means for rapidly recording large amounts of information in a small area in such a way that the information is read back with a high signal-to-noise ratio. These elements employ a thin recording layer of a certain metallic or organic material deposited on a support. The recording means comprises a beam of high energy-density radiation, such as a laser beam.
Usually, the laser beam is modulated, e.g., pulsed corresponding to a pattern of information, and focused onto the surface of the recording layer. The recording layer has sufficient absorbence at the wavelength of the laser beam so that the laser transfers enough energy to small portions of the layer to burn, evaporate or otherwise remove the metallic or organic material from those portions. This technique is usually referred to as ablative recording. Generally, there is continuous relative motion between the laser and the layer so that, as the laser is pulsed or modulated, discrete pits or holes of varying sizes are created in the layer. The sizes and spacings of these holes constitute the encoded information. This recorded information is usually read back by turning down the power of the writing laser or by using another laser of lower power, thereby precluding the reading laser from further physically altering the recorded layer. The reading beam, which is disposed to follow the same path as the recording beam, must also be significantly absorbed by the recording layer so that an optical density difference is detected between pits and unrecorded areas. This density difference is detected by a photodetector positioned to receive laser light reflected from the underlying support where holes have been formed in the recording layer (in the case of a reflective support) or positioned to receive laser light transmitted through the underlying support where holes have been formed in the recording layer (in the case of a transmissive support). The detected density variations are converted back into an electrical signal corresponding to the information recorded.
It is readily apparent that the designer of an ablative recording element is faced with a dilemma. While it is desirable that the recording layer have "high" absorbance during recording, a "high" absorbance during reading increases the risk of physically altering the layer, thereby damaging the recording. While turning down the power of the read laser avoids damage, this expedient usually decreases the signal-to-noise ratio of the playback. Thus, the absorbance of the recording layer for an ablative process is a compromise. The absorbance must be high enough to provide for recording using reasonable writing power, yet not so high as to result in physical damage at reasonable reading power.
Several materials have been suggested for this density-difference-type of ablative recording. Bismuth appears to be a preferred metal for such applications, and a single organic material, 4-phenylazo-1-naphthylamine, has been disclosed to be useful in U.S. Pat. No. 4,023,185. Unfortunately, all of the materials previously known to be useful in such applications are coated in layers of the desired thinness on a support only by techniques which are relatively expensive and time-consuming, such as vacuum-evaporative deposition, a method taught in U.S. Pat. No. 4,023,185 to be the only successful technique for forming a useful 4-phenylazo-1-naphthylamine layer. It is, therefore, very desirable to provide a class of materials for the recording layer which is easily coated, e.g., from a solvent, by quick and inexpensive coating methods and still provide a high-quality, high information-density recording capability.
The present invention provides a recording element, and a process for writing and reading the element, having the desirable characteristics described above.